This invention concerns grinding machines and in particular a mechanism for controlling wheel infeed in dependence on angular orientation between work spindle and wheel spindle axes.
The angular relationship between two such spindles can be affected by forces produced by grinding, particularly during face grinding, and this can introduce inaccuracies in the grinding process.
In JP-A-63191559 there is disclosed a grinding machine in which a grinding wheel is mounted on a spindle which is also pivotally mounted perpendicular to its axis. A detector measures any deflection of the spindle and emits a control signal for correcting such deflection via a piezoelectric element.
U.S. Pat. No. 5573443 discloses a tool spindle arrangement for a grinding process in which the inclination of a ground workpiece is measured by a device, and the angle of the spindle axis is adjusted in response to a signal from the device.
It is an object of the present invention to provide a means to control precisely the angular orientation between two such spindle axes, so as to set and maintain the desired angular relationship between the two spindles and thereby increase the accuracy of the subsequent grinding process.
According to the present invention there is provided a grinding machine comprising a workpiece spindle, a grinding wheel spindle, first mounting means for mounting one of the spindles for pivotal movement about a first axis which is perpendicular to the rotational axis of said one spindle, a first servo motor operable to position said one spindle about the first axis, characterised by sensors capable measuring distance which enable the position of said one spindle to be accurately determined relative to the other spindle by generating control signals for controlling the first servo motor.
Advantageously the machine may further comprise second mounting means for mounting either one of the spindles for pivotal movement about a second axis which is orthogonal to the first axis, and a second servo motor operable to position said either one of the spindles about the second axis, said sensors likewise controlling the second servo motor.
The second mounting means and the second servo motor may then be provided on the other spindle.
Preferably displacement sensors are employed.
Preferably the information obtained from the sensors is such as to allow the angle between the two spindles to be accurately determined.
The sensors may be positioned at three discrete points with respect to the spindles.
The measurements at the three discrete points are preferably made continuously; alternatively they may be made on a repetitive basis.
Measurements may be made at more than three positions.
It is possible to use relative changes in the measurements to calculate a change in angle between the two spindles and thereby allow the control signals to be computed for adjusting one or both of the servo motors to compensate for the movement which has been detected and reposition the spindle axis so as to maintain the desired relationship between the two spindle axes.
Angular movement of either spindle about its axis can be used to derive the change in linear distance between the two spindles resulting from the angular movement, and wheel feed control signals can be adjusted or corrected to compensate for any noted linear movement between the two spindles so as thereby to control precisely the infeed position between wheel spindle and work spindle.
In one arrangement each spindle assembly is supported in trunnion bearings mounted within a support so as to be pivotable about one of the said two orthogonal axes.
In an alternative arrangement each spindle assembly is supported by flexures which respectively define one of the two orthogonal axes about which the spindles pivot.
One of the orthogonal pivoting axes may be vertical and the other horizontal.
According to another aspect of the invention there is provided a face grinding machine in which a grinding spindle and a workpiece spindle are carried by respective housings, the workpiece and wheel spindle axes being parallel, characterised in that three probes are mounted for determining the distance between the two housings, each probe having associated therewith a sensor for generating a signal indicative of the distance between the two housings at the positions of the probes, and further comprising a servo motor responsive to the signals generated by the sensors for positioning one spindle relative to the other spindle.
Distance proportional signals may be digitised and the resulting digital signals are supplied to digital computing means programmed to compute from the digital signals the relative angular movement if any, between the two spindles as indicated by the signals from the probes, and to generate control signals for adjusting the servo motor associated with one or both of the two spindles to correct for any angular displacement detected, and further programmed to compute from the digital information from the sensors and/or from the control signals derived there from for controlling the servo motor or motors, correcting signals for adjustment of the wheelfeed control signals for adjusting the indeed position between wheel spindle and work spindle to compensate for any shift in the relative position of the two spindles.
The signals from the sensors need not necessarily be digital, and could instead be analogue signals, with appropriate gain modifiers.